The last time I went touring, many years ago, the only available cassettes were 5-speed. Combined with a double front chainwheel, this gave me 10 gears, although only 8 were actually usable (because of chain angle). In fact, as far as I remember, I only regularly used 6 of them on the tour. My question is, given that there are now 9 and 10-speed cassettes, is it feasible to go touring with just a single chainwheel? Has anyone here done that over any significant distance?

The last time I went touring, many years ago, the only available cassettes were 5-speed. Combined with a double front chainwheel, this gave me 10 gears, although only 8 were actually usable (because of chain angle). In fact, as far as I remember, I only regularly used 6 of them on the tour. My question is, given that there are now 9 and 10-speed cassettes, is it feasible to go touring with just a single chainwheel? Has anyone here done that over any significant distance?

It depends on how low you can get the cassette. If the chainring were small enough to give you at least a 30 inch gear, it might be doable.

The advantage of running two (or three) chainrings is the range of possible gears rather than the number of possible gears. The same cassette can get you much lower and higher gears with multiple chainrings. I run a 1x8 drive train on my cross bike, and it sees some occasional light touring (at least until I have a dedicated touring bike). While I have eight possible gears, just like a 2x5 drive train, a 2x5 has a much broader range of gears because of the two chainrings. If I didn't race the cross bike, I'd have two chainrings on it to make it easier to go up hills with gear and faster on the flats. It's completely possible to run one chainring, but I would rather have the bigger range of gears.

The other thing about running one chainring is that unless you run it single speed or fixed, you ought to have some sort of system to avoid dropping the chain. Either chainguards or a derailleur or whatever you come up with. Dropping the chain sucks, and putting it back on sucks too. An extra chainring and a derailleur solves the problem of the dropped chain and gives you a bigger range of gears. Ideal for touring.